Variable electrical resistor and method of manufacture



April 5, 1955 A. M. DAILY EI'AL 2,705,749

VARIABLE ELECTRICAL RESISTOR AND METHOD OF MANUFACTURE Filed May 13, 1952 2 Sheets-Sheet l INF-UP I IIIIIIIII IIIIIIII II PROVIDED WITH CONVENTIONAL RESISTANCE ELEMENT VARIABLE RESISTOR PROVIDED WITH RESISTANCE ELEMEN (1- Ti INVENTION SAME POWEIZ 7%. 4'. DISSIPAT ION CAPACITY CONDUCTIVE coAT- a SILICONE IzEsIII R E O A /7 M17 CONDUCTIVE PIGMENT P IM Clk} M m za 2? PLUS DIATOMACEOUS MATERIAL n Ur :7] y i,/ :;;3/:,3;; GLASS FIBER REINFORCED I 5/4 /72? [am/7mm" BACKING COAT- SILIQONE I2EIN [LS DIATOMACEOUS MATERIAL (W United States Patent VARIABLE ELECTRICAL RESISTOR AND METHOD OF MANUFACTURE Arthur M. Daily and Glenn M. Cocanower, Elkhart, Ind., assignors to Chicago Telephone Supply Corporation, E khart, Ind., a corporation of Indiana Application May 13, 1952, Serial No.

11 Claims. (Cl. 201-55) wherein the resistance element This invention relates to variable resistors of the type on the order of 400 F., and under humidity. purpose or objective of this invention is to conditions of extreme Another or altering their physical design or mode of construction from that previously Variable resistors of atures.

This invention not only makes ment of can have its wattageratmg increased more than fourfold, and that diameter of only /4" has a power dissipating greater than that heretofore possessed by the 1 /8 diameter unit.

In the evolution of this invention the use of ceramics r the b ase to which the conductive coating is applied Moreover, the discovery or attainment of a suitable material for the base of the resistance element would solve only part of the problem.

that the conductive coating, which capacity 2,705,749 Patented Apr. 5, 1955 2 and which often subject the base to great thermal shock will not create disruptive stresses in the conductive coating which might affect the stability of its resistance or the smoothness of its surface.

he solution to all of these to reside in thermal shock within the base.

Silicone resins is the term generally applied by the art to thermo-setting resins composed essentially of organosilicon polymers. The finely divided conductor, often referred to as a conductive pigment, may be graphite, carbon black or the like.

Thermo-setting high heat resistant resins in reinforced cly stifi' sheet form suitable for use in the making thing heretofore available.

in general, therefore, the invention contemplates subjecting the filler material such as its equivalent to fill and close the interstices which are base and assures a smooth contact surface. While a description of the invention without benefit of the entire rotating assembly in a position at which a greatly enlarged scale and with the surface irregularities of its base exaggerated;

Figure is an end view of two different s1zed resistors of the same type to illustrate one of the advantages of this invention, namely, size reduction without sacrificing ability to dissipate power; and

Figure 6 is a block diagram depicting one specific example of the procedure followed in the production of resistance elements in accordance with this invention.

Since the construction of the variable resistor generally, forms no part of this invention, a detailed illustration or description thereof is unnecessary. Hence, many of the details of construction are not illustrated in the drawing which is intended merely to depict the type of resistor with which this invention is concerned. hus it is sufficient to observe that the resistor comprises a flat terminal head 5 of insulating material to the front of which a mounting bushing 6 is secured. This bushing provides the means by which the resistor is mounted and also affords a bearing in which the operating shaft 7 of the unit is journalled. The inner or back face of the terminal head has a resistance element indicated generally by the numeral 8 mounted thereon and held in place by rivets 9 which pass through terminal extensions 10 on the resistance element, the terminal head 5 and terminals 11.

As best shown in Figure 3 the resistance element 8 is a split ring with the terminal extensions 10 projecting radially outwardly from its ends. Since the invention resides in the specific construction and composition of the resistance element it will be discussed in detail hereinafter.

At this juncture it should be observed, however, that a collector ring 12 is encircled by and positioned coaxially of the resistance element, and that an insulating washer is interposed between the underside of the collector ring and a pair of tangs extending radially inwardly of the inner edge of the resistance element. In this manner the collector ring acting through the washer 13 and the tangs 15 coacts with the rivets 9 to hold the resistance element fiat against the terminal head. The collector ring 12 is, of course, of metal and has a terminal extension 1 projecting outwardly therefrom and suitably fastened to the terminal head.

The rotatable shaft 7 protrudes through the terminal head 5 and mounts a contactor assembly indicated generally by the numeral 17. This contactor assembly includes an insulated drive head 18 and a metal stop plate 19 both nonrotatably secured to the end of the shaft, and resilient contactor fingers 20 and 21 which ride respectively on the surface of the resistance element and the collector ring.

A C-washer 22 engaged in a groove 23 in the shaft and bearing against the adjacent end of the bushing holds its spring fingers 20 and 21 are under tension, and as is customary the unit is protected by an enclosing housing 24 which preferably is in the form of a stamped sheet metal cup secured to the terminal head by tangs 25.

The shape of the resistance element 8 and its manner of attachment to the terminal head follows the practice more or less well established in the art and thus forms no part of this invention, but the composition of the resistance element, i. e. the physical make-up of its base 26 and is conductive coating 27, is all important.

The conductive coating 27, as is the practice in the production of resistors of the type here under consideration, is painted, sprayed or otherwise deposited on the base. and since the specific manner in which the coating is applied, that is, whether it is painted on, sprayed on, or otherwise deposited, is not critical as long as it is deposited in liquid form, it is to be understood that when the term painted on is employed herein and in the claims any one of the possible ways of applying the coating in liquid form is meant.

A specific example of the way in which the resistance element of this invention is produced is diagrammatically illustrated in the chart, Figure 6. The high heat resistant reinforced resin material selected for the base, which, for purposes of illustration, may be silicone resin reinforced with glass fibers and which is obtained in sheet form of the desired thickness, from the manufacturer, is first cut into appropriate sized pieces or strips and then heat-cured at a temperature of 600 F. for fifteen (15) hours.

Next, as indicated on the chart, Figure 6, a backing coat is applied to one face of the heat treated material. This backing coat consists essentially of silicone resin and any suitable filler, in a hydro-carbon or other suitable solvent.

After the backing coat is applied the sheets or strips are then subjected to a drying heat to remove the tackiness of the backing coat. This done, the opposite face of the sheets or strips have a prime coat applied thereto which consists essentially of silicone resin and a filler of diatomaceous earth or its equivalent.

With the prime coat applied, the sheets or strips are then subjected to a rather extended heat curing treatment consisting of three stages-the first at 340 F. for thirty (30) minutes; the second at 460 F. for thirty (30) minutes; and the third at 480 F. for sixteen (16) hours.

Upon completion of this heat treatment the conductive coating is applied to the sheets or strips over the prime coat. This conductive coating comprises essentially silicone resin and a finely divided conductor such as graphite or carbon, in a suitable solvent, preferably a hydro-carbon.

With the conductive coating applied the sheets or strips are again subjected to a three stage heat treatment similar to that given them after the application of the prime coat, and upon completion of this heat treatment a secon coat of conductive material is applied over the first, the second coat preferably being of the same composition as the first.

Thereafter, the sheets or strips are heat treated only sufficieutly to remove the tackiness of the second and final conductive coat.

All that remains following drying of the final conductive coat is to punch out the resistance elements, to the shape shown in Figure 3, and give them an additional final three stage heat treatment the same as the previously mentioned treatments.

As noted hereinbefore, resistance elements made this way have been found to perform entirely satisfactorily under the most severe conditions of humidity and at temperatures far above those which could be withstood by resistors of this general type heretofore available. The smooth, hard contact surface of the resistance element retains its ability to withstand the abrasive effect of the metal contact finger sliding thereover even under conditions of very high humidity coupled with temperatures on the order of 400 F. But what is perhaps one of the most significant aspects of the invention, as depicted in Figure 5 of the drawings, is that a tiny, miniaturized unit of 4" overall diameter equipped with the resistance element of this invention has a higher power dissipating capacity than that of the much larger 1 /3 in diameter unit having the conventional resistance element; and that as a corollary the invention enables greatly increasing the wattage rating of resistors of any given size.

It follows, of course, that the ability of the resistance element per se to withstand high heat would be of little value if the rest of the insulation in the resistor were not also capable of withstanding such elevated temperatures. Hence, the terminal head 5, the contactor carrier 18 and the spacing washer 13 must also be made of high heat resistant material. Any one of the mentioned thermosetting high heat resistant reinforced resins can be employed for this purpose.

By curing the base material at a temperature far above the maximum that will be encountered in use of the resistor, the material is dimensionally stabilized; but as a result of this severe heat treatment and the consequent shrinkage of its resin content, its inherent rough surface texture is aggravated. However, the prime coat of silicone resin and diatomaceous earth, or its equivalent, completely seals and closes all the tiny interstices and depressions in the heat-treated base material and provides a smooth surfaced foundation for the conductive coating. In addition, this prime coat also serves to cushion the conductive coating against the consequences of thermal shock which might take place in the base as a result of extreme changes in ambient temperatures, and since it seals and closes the areolar surface of the base material and provides a smooth surfaced foundation for the conductive coating, it affords assurance of uniform resistivity in the conductive coating which would not be had if the coating were applied directly to the base material. Of course, where extremely close electrical tolerances need not be held, the intermediate film provided by the prime coat can be omitted, but in that case it may be necessary to incorporate more of an elastic filler at least in the first of the two conductive coatings applied.

des1red electrical conductivity, a

the outer conductive coating, lm consisting essentially of silicone resm and bondlng the outer conductive coating to the presence of high heat and despite severe ambient temperature changes.

2. The variable resistor of claim 1 further characterized by the fact that the intermediate film contains a filler of diatomaceous material.

3. A variable electrical resistance device is slid thereover.

4. The variable electrlcal resistance device set forth in claim 3 wherein the base is reinforced with inorganic particles.

resistance variable electrical device set forth of a thermo-setting resin selected from the group conmelamine, phenolic and polyester sistor by sliding the outer electrically conductive coating.

e variable resistor set forth in claim 7 wherein said inner coating includes a filler of diatomaceous material.

9. The method of making a resistance element for a variable resistance dev1c e of the type wherein a selected as 400" F. material in sheet form to a a temperature well above 400 F. b higher than 600 F. to thereby permanently shrlnk the resin and dimensionally stabilize the base material; applying a prime coat consisting essentially of silicone resin to one surface of the fissures therein duration at thereby permanently shrink th stabilize the References Cited in the file of this patent UNITED STATES PATENTS 2,258,218 Rochow Oct. 7, 1941 2,549,516 Parry Apr. 17, 1951 2,559,077 Johnson et a1. July 3, 1951 

